In Japan, okara (bean curd refuse) is by-produced about 0.8 million tons per year, but most of them are used for feed or fertilizer with low added-value. As one of the methods to make good use of such okara, extraction of various materials having high functionalities from okara has been tried. For example, Patent Document 1 describes that a water-soluble soybean polysaccharide is obtained by degrading water-insoluble vegetable fibers containing a protein under acidic conditions around the isoelectric point of the protein at a high temperature.
Although aggregation and precipitation of a milk protein usually occur at a pH of 5.0 or less, addition of this water-soluble soybean polysaccharide enables to stabilize the dispersion of a protein even under an acidic pH from 3.8 to 4.2, and thus to produce an acidic milk beverage having a low viscosity (Patent Document 2). However, these water-soluble soybean polysaccharides produced by the conventional method have not shown an ability enough to stably disperse a protein, for example, in a pH range around the isoelectric point of a milk protein or at a pH exceeding such isoelectric point, i.e. at a pH of 4.2 or more.
Meanwhile, pectin and carboxymethylcellulose other than water-soluble soybean polysaccharides have been used as a dispersion stabilizer for a protein under acidic conditions. When these substances are used as a stabilizer for acidic milk beverages, a milk protein can be dispersed and stabilized in a pH range around the isoelectric point of a milk protein, i.e. in a pH range of about 4.2 to 4.8, which is a pH range such that stabilization with the water-soluble soybean polysaccharides is difficult, but the beverage produced have viscous or pasty mouth feel. It is difficult to obtain a beverage having a low viscosity in a pH range around the isoelectric point of the milk protein.
A milk protein has a positive charge under acidic conditions at or below the isoelectric point. Studies on the model of dispersing protein in acidic milk beverages using polysaccharides, particularly pectin and a water-soluble soybean polysaccharide, has been advanced (Non-Patent Document 1). It is thought that aggregation and precipitation of proteins are suppressed due to electric repulsion and steric hindrance by ionic and hydrophobic bonding to the protein surface via own negative charges of polysaccharides which distribute to dispersion stability of a milk protein. In order to improve dispersion stability of polysaccharides, a lot of approaches to increasing the negative charges of the polysaccharides have been made so far. For example, carboxymethylcellulose as mentioned above is increased negative charges by introducing carboxymethyl group into cellulose. However, a stabilizer which enables to stabilize the dispersion of a milk protein at a low viscosity in a pH range around the isoelectric point of the milk protein has not been obtained yet. Moreover, with respect to the dispersion stability of a protein, investigation focused on the molecular mass of stabilizers has not been known so far.    Patent Document 1: JP 2599477 B    Patent Document 2: JP 3280768 B    Non-Patent Document 1: J. Agric. Food Chem., 54(17), 6241-6246, 2006